The present invention relates generally to a weathering test apparatus of the type used to expose test specimens to solar radiation and other weathering effects on an accelerated basis, and more particularly, to such an improved accelerated weathering test apparatus that provides an automated soaking cycle regardless of the time of day.
Manufacturers of exterior coatings, such as paints and finishes, as well as plastics and other components which tend to degrade under exposure to solar radiation and other weathering effects, often want to know how such products will perform following years of exposure. However, such manufacturers typically require such information in a much shorter time than it would take to exposure such materials to weathering effects under normal conditions. Accordingly, accelerated weathering test apparatus have been developed which accelerate the effects of weathering due to outdoor exposure in a much shorter time so that manufacturers need not actually wait five or ten years in order to determine how their products will hold up after five or ten years of actual outdoor exposure.
One conventional outdoor accelerated weathering test apparatus is disclosed in U.S. Pat. No. 4,807,247 issued to Robins, III, and shown in FIG. 1. The aforementioned test device includes a Fresnel-reflecting solar concentrator having a frame 32 with a series of ten flat mirrors 34,36 which focus natural sunlight onto a series of test specimens secured to a target board measuring approximately six (6) inches wide by fifty-five (55) inches long. The Fresnel-reflecting solar concentrator directs solar radiation onto the target board area with an intensity of approximately eight suns. Both the bed 32 which supports the mirrors 34, 36 of the solar concentrator, and the target board, are supported by a frame which can be rotated to follow daily movements of the sun. A solar tracking mechanism responsive to the position of the sun, controls the operation of an electric motor used to rotate the test apparatus to follow movements of the sun.
The axis of rotation of the test machine is oriented in a north-south direction, with the north elevation having altitude adjustment capability to account for variation in the sun""s altitude at various times during the year. Such known testing apparatus are also provided with an air tunnel 44 mounted above the target board. An air deflector causes air escaping from the air tunnel to be circulated across the test samples mounted to the target board to prevent the test samples from overheating due to the concentrated solar radiation to which they are exposed. A squirrel cage blower 48 communicates with the air tunnel 44 for blowing cooling ambient air there through. Devices and mechanisms have been adapted to control the blower in order to maintain the temperature of the target test samples substantially constant during daylight hours despite variations in the daytime ambient air temperature, and despite variations in the solar radiation intensity. In addition, water spray nozzles 60 are provided proximate to target board for wetting the test samples at periodic intervals to simulate the weathering effects of humidity, dew, rain, etc.
Standardized testing methods have been developed for operating outdoor accelerated weathering test apparatus of the type described above. The American Society for Testing and Materials (ASTM) has issued standards such as, but not limited to, standard G90, which is directed to testing procedures and operating parameters for conducting such outdoor accelerated weathering tests. Other standards have been developed by the Society of Automotive Engineers (SAE), Ford, International Standards Organization (ISO), American National Standards Institute (ANSI), Japan Industrial Standard (JIS) and other such standards organizations which are directed to accelerated weathering tests. However, no standard has been developed which incorporates a soaking cycle into an outdoor, natural light accelerated weathering test cycle. Standard D5722 was developed for accelerated finish failure involving loss of film integrity, such as cracking, peeling and flaking of factory-coated embossed hardboard. This Standard accelerates long-term weathering effects by subjecting the test specimens to concentrated natural sunlight (with optional periodic daytime surface water spray) plus a remote soak-freeze-thaw cycle. The Standard procedure requires removing the target board with attached test specimens from the testing apparatus and immersing the target board in a deionized water soak tank maintained at 21xc2x0 C.+/xe2x88x923xc2x0 C. (70xc2x0 F.+/xe2x88x925xc2x0 F.) for one hour. After soaking, the target board with attached test specimens is placed in a freezer maintained at xe2x88x9218xc2x0 C.+/xe2x88x923xc2x0 C. (0xc2x0 F.+/xe2x88x925xc2x0 F.) for 12 hours. The following morning, the test specimens are allowed to thaw for a minimum of one hour under laboratory ambient conditions. The target board with attached test specimens is then remounted on the exposure device in accordance with prior practice.
It has been recognized by independent study that a soaking cycle is a significant and important variable for evaluating degradation of materials in accelerated weathering tests. The inventor hereof has conducted numerous experiments in the field of accelerated weathering testing which verify and confirm that a soaking period is a significant and important variable in such testing. The results of these experiments were discussed in technical papers prepared, published and presented by the inventor titled xe2x80x9cFractional Factorial Approaches to Emmaqua Experimentsxe2x80x9d and xe2x80x9cApplying Taguchi Designs to Emmaqua Weathering Experiments.xe2x80x9dBoth papers document the results of the background research related to different techniques for accelerated weathering tests which concludes that immersion plays a critical role in weathering degradations.
Apart from outdoor accelerated weathering test devices of the type described above, other test devices are also known which utilize an artificial source of radiation and immersion in water to exposure test samples. An example of such a test devices are disclosed in U.S. Pat. No. 3,116,977, issued to Grabowski, et al.; U.S. Pat. No. 3,224,266, issued to Klippert; U.S. Pat. No. 3,266,306, issued to Arnold, et al.; U.S. Pat. No. 3,685,969, issued to Young III; U.S. Pat. No. 3,936,273, issued to Powell; U.S. Pat. No. 4,012,954, issued to Klippert; U.S. Pat. No. 4,282,181, issued to Pierce; U.S. Pat. No. 4,698,507, issued to Tator, et al.; and other conventional testing methods.
U.S. Pat. No. 3,116,977 discloses an apparatus to screen corrosion inhibitors by immersing metallic test specimens into a heated bath of water having the inhibitor dissolved therein, withdrawing the metallic specimens and heating them and continuing the periodic immersion and withdrawal for a substantial period. There is no provision for exposure to solar radiation or other light source.
U.S. Pat. No. 3,224,266 claims an apparatus for testing samples under conditions such as humidity, rain, or complete immersion in liquid, heat and air circulation as well as light and dark periods with controlled changeover between light and dark. It provides for heating with a heater source of warm air in addition to heat from the illumination source. This prior apparatus does not provide for exposure to solar radiation or rapid sequencing of immersion followed by exposure to solar radiation.
U.S. Pat. No. 3,266,306 describes an apparatus to test resistance of materials to humidity by exposing them to steam pressure in a chamber. No solar radiance exposure or other light source exposure is practiced in this prior art patent.
U.S. Pat. No. 3,685,969 discloses an apparatus for testing the strength of specimens under corrosive conditions. Specimens under stress are subjected to intermittent immersion using gravity flow of a corrosive fluid to and from a fixed tank with the specimens. Solar radiation or other light source exposure is not mentioned.
U.S. Pat. No. 3,936,273 discloses an apparatus for determining the corrosion protection performance of a fluid. This apparatus rotates test specimens mounted on a shaft through the liquid and thereafter may maintain this test specimens for extended periods in the fluid prior to examination for the degree of corrosion. This prior art patent does not provide for solar radiation or other light source exposure.
U.S. Pat. No. 4,012,954 claims an apparatus for testing light-and weather-resisting properties of materials by employing a mirror reflecting infrared and passing ultraviolet and visible light from an illumination source together with a second mirror reflecting visible and ultraviolet light while transmitting the infrared portion. Samples on a horizontal support can be flooded with water, drained or water cooled, and may also be air cooled. In this prior art patent, there is no exposure to solar radiation.
U.S. Pat. No. 4,282,181 describes an apparatus for accelerating corrosion testing of parts in which the parts are lowered into a corrosive medium and then raised into a drying zone for predetermined, repetitive periods. No mention of solar radiance or other light source exposure is made.
U.S. Pat. No. 4,698,507 describes an apparatus for testing for resistance of immersion swelling, drying shrinkage, thermal expansion and thermal contraction under light exposure. The test samples are placed on a mount on a rotating shaft which immerses the sample in water, heats and dries it, and exposes it to light before cooling it by again immersing it in water. The corrosion resistant chamber enclosing the rotating shaft is composed of a lower tank base and a cover fitted with fluorescent lights and infrared heating strip, a thermocouple and a viewing port. The test liquid is maintained at a constant temperature by fluid flow through a heat exchanger using a thermocouple and a controller. In this prior art patent, there is no exposure to solar radiation.
While such test devices have the advantage of permitting precise control over radiation intensity, temperature, and humidity, such test devices fail to duplicate the actual light spectrum of natural sunlight to which the samples under test will actually be exposed in everyday use. It has been acknowledged and recognized that the outdoor (natural) light source and indoor (artificial) light source test apparatus are distinct from one another and provide different sets of empirical data. For example, ASTM has issued Standard G26 and SAE has issued Standard Test Method J1960 for operating an artificial light-exposure apparatus (Xenon-Arc type) with and without water for exposure of nonmetallic materials.
Outdoor accelerated weathering test devices of the type described above in regard to U.S. Pat. No. 4,807,247 (FIG. 1), have the advantage of using natural sunlight and hence the samples under test are exposed to the actual spectrum of sunlight. However, one disadvantage of outdoor accelerated weathering test devices has been discovered, namely the inability to include automated soaking cycles into a test method.
Conventional testing methods which include a soaking cycle are disadvantageous in that they are very time and labor intensive. Accordingly, a very limited number of exposure and immersion cycles can be completed with the requisite degree of reliability within a certain period. The procedure for the conventional testing method includes exposing the test specimens to concentrated sunlight as per ASTM G90 or other acceptable standard operating procedure. Generally, the steps of the soaking test cycle procedure include: (1) mounting specimens in frames; (2) rotating the apparatus to an inverted position to provide access to the target board area; (3) centering the frames with specimens on the target board; (4) attaching the frames to the target board using screws or other appropriate attachment mechanisms; (5) activating the apparatus cooling air blower; (6) activating the apparatus solar tracking system; (7) rotating and focusing the apparatus to provide solar irradiance on the specimens; (8) tracking the sun throughout the daytime; (9) deactivating the blower and tracking system in the evening; (10) rotating the apparatus to the inverted position to provide access to the target board area; (11) removing the attachment mechanisms from the frames; (12) removing the frames and specimens from the apparatus; (13) transporting the frames into the laboratory where the soak tanks are located; (14) filling a tank with de-ionized water; (15) submerging a heating element in the tank; (16) activating the heater element to warm the water; (17) inserting a temperature probe into the water which is connected to a heater controller for controlling the heater element and water temperature; (18) setting the water temperature controller to a predetermined set point; (19) immersing the specimens in the water contained in the soak tank; (20) immersing the specimens in the soak tank for predetermined period of time (usually overnight); (21) removing the specimens from the soak tank after soaking for a predetermined amount of time; (22) transporting the specimens from the soak tank in the laboratory outside to the apparatus; and (23) reattaching the specimens in the frames to the apparatus target board using screws or other appropriate attachment mechanisms. This cycle of irradiance-specimen removal-specimen immersion-specimen remounting-irradiance is continued for a predetermined number of cycles.
There are several disadvantages of this conventional soaking cycle test method. One disadvantage is that a technician must dismount the frame and test specimens from the target board prior to the immersion step. This would be required hundreds of times throughout a single exposure test (hundreds of cycles). The dismount-remount steps are labor-intensive, expensive and inefficient. Further, the technician is required to remount the frames and test specimens onto the target boards after the immersion step. Again, this may be required hundreds of times for a single exposure test which is labor intensive, expensive and inefficient. Another disadvantage is that manual removal and remounting requires lengthy periods of time. Further, manual dismount and remount is subject to mistakes such as improper positioning of test specimens with respect to cooling air, irradiance or correct machine, all of which introduce sizable errors into the test cycle and skew the results of such an expensive test. Another disadvantage is that the amount of time required to dismount and remount specimens makes multiple immersion cycles within a 24-hour period highly unattractive for experimentation or testing. In the prior art, during periods of solar irradiance (daytime) only water spray was used to provide moisture to the test specimen surface while the test specimens were mounted on the target board. Yet another disadvantage is that the conventional soaking cycle method requires transportation of specimens from the apparatus target board to remotely located soak tanks (currently inside laboratories). Thus, increasing the probability of damage to specimens due to handling and consequently introducing errors and skewing the accuracy of the test results.
Prior outdoor accelerated weathering test apparatus only provide for spraying of water on the test specimens during periods of solar irradiance. However, soaking periods have been determined to be important and significant with regard to degradation of materials. Incorporating a soaking period in the conventional irradiance cycle test method is labor intensive, expensive, and inefficient. The conventional soaking cycle test method introduces numerous errors into the experimentation process which skew the results of the exposure test preventing reliable interpretation of the test results. Consequently, there exists a need for an improved accelerated weathering test apparatus and method incorporating a soaking cycle.